It’s a lifesaving treatment for some, but radiation therapy can carry risks that last for decades.

Researchers in Europe report in the New England Journal of Medicine that women with breast cancer who were treated with radiation slightly increased their risk of dying from heart disease or a heart attack in the years following their therapy. For most women with no history of heart disease, the risk was small — about 2% for having a heart attack in the 30 years following radiation treatment — too small to trump the benefits of radiation in controlling cancer, doctors say.

Since previous reports hinted at potential increased risk of heart problems among women radiated in the chest area, the scientists studied the records of 2,168 women who had radiation for breast cancer from 1958 to 2001 in Sweden and Denmark. Among the participants, 963 had “major cardiac events”— like a heart attack — at some point in the 30 years after their breast-cancer radiation treatment. The researchers also estimated the dose of radiation to which the women’s hearts were exposed.

The heart risks increased within a few years after exposure and continued to increase for several decades. There was some risk at the lowest level of exposure, but the higher the dosage, the higher the risk overall. Most women in the study received about 4.9 units, called grays, of radiation to the heart as part of their breast-cancer treatment. With every increase in gray, the risk of heart problems, including compromised blood flow to the heart and heart attack, increased by 7.4%.

The results, while sobering, also need to be considered in historical context. Doctors say that improved technology has led to lower doses of radiation to treat cancers today (some of the women in the study were treated in the 1960s). Dr. Nancy Snyderman, the chief medical editor at NBC News, said in a segment on NBC News that radiation doses are now both smaller and more precise, and that procedures today can largely avoid exposing the heart.

That’s why experts, as well as the study’s authors, stress that the risk of radiation treatment does not outweigh the benefits in helping women to survive breast cancer. “It would be a real tragedy if this put women off having radiotherapy for breast cancer,” lead author Sarah Darby, a professor of medical statistics at the University of Oxford, told the New York Times.

“Women should not walk away from this thinking they should not get radiation. It is an integral part of treatment, and it is the reason that right now in the U.S. over 90% of women with breast cancer are cancer-free within five years,” says Dr. Javid J. Moslehi, a co-director of the cardio-oncology program at the Dana-Farber Cancer Institute in Boston, who wrote an editorial accompanying the study. “There are 3 million survivors today and that number is due to the development of these treatments.”

To lessen their risk, all women, including those who may need radiation treatment for breast cancer, should focus on the heart-disease risk factors that are within their control: eating a healthy diet, getting plenty of exercise and not smoking, for example. But now that researchers and clinicians have stronger evidence of the connection, doctors can pay more attention to breast-cancer patients who also have a history of heart disease or heart-disease risk factors. “This also introduces a new field we didn’t know existed years ago,” says Moslehi. “Now more people are surviving and some are having heart risks. It’s a new area of study.” With new opportunities for improving survival as well.